Polymer fibers of a wide variety of materials are of course well-known. Extrusion is much the most common process for producing such fibers. In the usual extrusion process, molten polymer is forced under relatively high pressure, typically on the order of hundreds of psi, through an extrusion die that is comparatively close in dimension to those of the finished fiber. More specifically, some swelling of the fiber may occur as it exits the die, and the fiber can be drawn down by application of tension. It is of course well-known to apply a polymer sheath to a substrate, e.g., to insulate a metallic wire, by passing the wire through the extrusion die and extruding the polymer sheath around the substrate.
In most cases homogenous fibers or sheaths, that is, manufactured of a single polymer material, are adequate for the intended purpose. Where a multiple-layer product is desired, that is, with a central core of a first polymer sheathed in a layer of second polymer, or where a wire or another substrate is to be coated in two different layers of polymer, this is typically accomplished by first forming a monofilamentary fiber, or sheathing a metallic substrate, in a first extrusion step and then adding the outer sheath in a second extrusion step, thus requiring multiple extruders and/or multiple cross heads. It would be difficult, at best, to make a filamentary product having two or more distinct polymer components in a single extrusion operation, as the pressure required to force the polymer materials through the extrusion die would cause the polymer materials to tend to become admixed with one another.
The present invention seeks to avoid these limitations on the conventional processes of forming filamentary polymer products.